Not sure of the science, something to do with bacteria growth and temperatures. Basically raised with it as a golden rule from childhood. Grandfather was Chief Health Inspector in Glasgow, Great Aunt in Army catering, father a GP, aunt was in home economics as a teacher, I remember it from school myself plus if you check out food hygiene sites you see it there as well(eg
http://www.eatwell.gov.uk/keepingfoodsafe/cook ing/).
OK, seeing as I'm rapidly coming on 50 myself and was a (very) late addition to my family it goes without saying my grandparents and parents generation didn't have microwaves, but it is to do with temperature changes rather than the methods of heating employed.
As I said though, no guarantee you will get food poisoning by doing it, but you are increasing the chances of it. You pays your money and takes your chances.